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  1. #1
    I just talked to a well-recommended HVAC company who told me he NEVER would use mastic sealant on sheet metal or duct connections inside an attic or house, since it's made of tar and is carcinogenic.

    Has anyone else heard this?

  2. #2
    Join Date
    Feb 2004

  3. #3
    Join Date
    Mar 2005
    oil based will off gas! Stick with the water based! TAR WHAT THE WHATA

  4. #4
    Join Date
    May 2004
    Rapid City, SD
    Maybe something they used 50 (or more) years ago was bad for you (ohhhhh, say asbestos for example). But everything today is 100% safe. With all of the people being as concerned about indoor air quality now-a-days, there's no way duct sealer would be bad for you.

    What is bad for you is unsealed ducts. It brings in contaminated and usually unfiltered air and you breath that.

  5. #5
    Join Date
    Feb 2004
    sounds to me as though he just does not want to take the time to do the job correctly.

    have you recieved any other estimates?

  6. #6
    Join Date
    Aug 2004
    North Richland Hills, Texas
    Modern water based latex duct sealing mastics are safer than the latex paint covering your walls.
    If more government is the answer, then it's a really stupid question.

  7. #7
    Join Date
    Nov 2004
    I peel a few avacados and make guacamole with it.No problem

  8. #8
    Join Date
    Mar 2002
    Richmond, Virginia
    Find yourself another "well recommended" company.
    There is nothing more frightful than ignorance in action....Mark Twain

  9. #9
    Thanks, everyone for your emphatic responses!

    billva, I was just asking him a few questions over the phone before I scheduled an estimate. I figured I could weed out the companies I didn't want that way. I'm not even sure if he was an installer himself.

    And yes, I've received other estimates. The difference between the highest and lowest bids seems to depend on whether or not they do a manual J; whether or not they use mastic sealant versus duct tape; whether or not they connect to existing vent pipe or install a new one; and whether the job takes one day or five. Since the most expensive companies use as a selling point the fact that they use mastic sealant -- I was surprised to get this guy's strong reaction to "run away" from anyone who uses it! BTW, this guy's ballpark estimate over the phone was way more than the highest I'd received so far. I'll need asbestos abatement as well, and the abatement company they recommend charges almost three times as much as the other companies do. Does that mean none of the others would do as thorough a job on the asbestos?!!

    drk, amickracing, mark beiser, and jacob --thank you for your reassurance.

    HVAC Pro, I'll keep looking. This company has a AAA rating with the BBB, no consumer complaints, and they've been in business since 1970. It's no wonder I thought they'd be good.

  10. #10
    Join Date
    Sep 2005


    Why mastic? why not silicone, or permatex gasket stuff or liquid nails or even some type of glue. Why mastic specifically?

  11. #11
    Join Date
    Sep 2001
    East Stroudsburg, PA
    The tar comment from this "contractor" comes from the fact that the word "mastic" has many meanings. In hvac, we use the rubberized mastic. No petroleum base, but the auto industry used a petroleum based "mastic" to secure windshields for many years.

    HVAC mastic is brushable, and remains flexible after many years of drastic temp changes, fills voids, and (the water-based stuff, at least,) is relatively non-toxic.

    In comparison, silicone breaks down after many years and has a foul odor until it cures, which could take days if it is cold out. It is also expensive compared to mastic. It is not brushable, either.

    Liquid nails is an adhesive, not a sealant.

    Permatex is a company that makes many different products, but the RTV gasket maker is simply a more expensive, higher temp silicone. Again, expensive, not brushable.

    You can buy a gallon of mastic for the same price as 2 10 oz tubes of a decent silicone sealer, and be able to wash your hands when you're done brushing it. Silicone makes a hell of a nasty mess, and must be applied with a cualking gun.

    Glue is, again, like Liquid Nails, an adhesive, not a sealant.

    Pick a contractor that is willing to seal your ducts. in addition to the rest of the installation being in line with contemporary professional standards. You could give that tar guy the url to this website, and if he is willing to learn the truth about the industry, we'll be more than happy to assist him.

    [Edited by condenseddave on 11-09-2005 at 02:33 AM]

  12. #12
    Join Date
    Feb 2003
    why do you think you need asbestos abatement? -- leave it alone if possible -- or cover it with alum foil & mastic
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  13. #13
    cem-bsee: Several contractors have told me the ducts have asbestos. By California law, an asbestos abatement co. has to come in and remove it (or encapsulate it where it's not practical to remove) before a new heater can be installed.

    I would rather pay extra for the safest sealant, as long as it's effective. The fact that mastic is "relatively non-toxic" scares me, as I prefer that all building materials in my house be as environmentally safe as possible.

    Approximately how many more years will mastic last compared to silicone? And if the silicone breaks down, can it be replaced without needing to replace the ducts too?

    Thanks so much for your information.

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